In known electrical interconnection devices, electrical connection between two conducting elements is achieved by bringing them into physical contact such that an ohmic connection is formed. Such an ohmic connection is characterized by a contact resistance. On a microscopic scale, current is transferred between the two conducting elements via randomly distributed load-bearing areas, referred to as a-spots, that form between the elements when they are in mutual contact. Increasing the mechanical force that brings the elements into contact tends to increase the number and size of the a-spots. Accordingly, the a-spots contribute a constriction resistance to the contact resistance, the constriction resistance being proportional to the mechanical force applied between the elements, where the constriction resistance is described by: EQU R.sub.s =.rho./na (Eq. 1)
where .rho. is the resistivity of the conducting element, n is the number of a-spots created, and a is the average linear dimension of the a-spots. Since the dominant conduction mechanism is ohmic, the contact resistance is constant over a wide voltage range.
It is common for an insulating surface film of lubricant, metal oxide, or other contaminant to be found on the contacting surface of one or both contact elements, thereby contributing an additional resistance referred to as an effective resistance. Consequently, the contact resistance is the sum of the constriction resistances and the effective resistances of the respective surfaces.
If the surface film is thin, i.e., less than 100 .ANG.ngstroms, some conduction will occur due to electron tunneling through the film. Such tunneling can occur by several mechanisms. As illustrated in FIG. 1, when a voltage is applied across the contacts, the Fermi level 10 of the metal constituting the positive contact is lower than the Fermi level 12 of the metal of the negative contact. Although electrons at the Fermi level 12 will not have enough energy to cross over the potential barrier 14 at the metal-film interface, there will be some probability that electrons will "tunnel" through this barrier, in accordance with Schrodinger's equation from elementary quantum mechanics. Thus, a small but measurable current will flow, despite the presence of the insulating surface film on the contact elements.
As the applied voltage over the surface of the contact elements is increased to, for example, 10.sup.6 V/cm, a second effect, known as field emission, takes place. This effect is described by the Fowler-Nordheim equation, which is approximately: EQU J.apprxeq.AE.sub.2 /.phi. exp[-B.phi..sup.3/2 /E] (Eq. 2)
where J is the current density, E is the electric field, .phi. is the work function of the material, and A and B are constants.
In present-day connector technology, the total tunneling current is a very small fraction, typically 10.sup.-6 to 10.sup.-3 of the total current carried by the a-spots. This is because the radius of curvature of the a-spots, typically 10.sup.4 .ANG. to 10.sup.5 .ANG., is large enough to promote ohmic conduction.
If the surface film is too thick to allow electron tunneling, another phenomena, known as "fritting" occurs. In high applied electric fields, such as fields greater than 10.sup.6 V/cm, electrons injected into the film due to field emission cause an avalanche breakdown of the film at the point of injection. A channel created by the breakdown causes localized heating of the contacts, which softens the metal surface and thereby causes an a-spot. This a-spot will widen as the current conducted through the a-spot increases. Note that fritting occurs at sites similar to those that cause tunneling, i.e., at protuberances at the contact surface, the protuberances serving to concentrate the electric field.
The presence of particulate contaminants, such as dust, on the surface of the conducting elements further increases the contact resistance. This occurs by imposing a barrier to complete contact closure. Referring to FIG. 2A, if the metal of the conducting element 16 is softer than the dust particle 18 in contact therewith, it will take an amount of force proportional to the hardness of the metal to deform the contact enough to cause complete contact closure between the element 16 and a complementary element 20, as shown in FIG. 2B. For a particle with a cross-sectional area A, the mechanical pressure P applied to the contact is: EQU P&gt;AH (Eq. 3)
where H is the hardness of the contact metal. For particles of 2 .mu.m.sup.2 -200 .mu.m.sup.2 area and H values of 10.sup.9 N/m.sup.2, P must be greater than 20 grams of force in order to insure contact. For devices with many concurrent contact elements, such as pin-grid arrays, such a high P value per contact element results in unacceptably high total applied pressure. For example, a 300 pin-grid array socket would require a total applied pressure of 6 kg to provide reliable contact.